Page 337 - Sensors and Control Systems in Manufacturing

Advanced Sensors in Pr ecision Manufacturing
dioxide laser emission and atmospheric physics. These sensors tend 293
to be more expensive than both silicon photodiodes and CdS photo-
conductors.

6.7.1 Integrated Circuits
Incorporating additional electronics directly onto a semiconductor
sensor chip makes it possible to add additional functions to the sen-
sor. An optical IC is an integrated circuit comprising photodiode and
electronic-signal-processing-circuits. Such additional functions as
current-to-voltage conversion and reference-level sensing (a Schmitt
trigger, for example) can be incorporated. Other optical ICs can pro-
vide signals highly immune to noise, such as a current-to-frequency
conversion.
The principal advantages of an optical IC are ease of use, small
size, and immunity to electronic noise compared to a photodiode
with separate electronics. Typically, these devices are much more
expensive and offer a very limited active light-sensing area. Custom
tooling for specific applications is also expensive.

6.7.2 Hybrids
The electronic functions of an optical IC can also be provided by a
hybrid circuit that has an unpackaged IC components “die” attached
to a substrate that also contains a photodiode.
This type of sensor combines the ease of use and immunity to
electrical noise of an optical IC with increased design flexibility and
lower tooling costs. In addition, the sensitivity can easily be increased
with a larger photodiode active area without the added cost of a sep-
arate detector. The primary disadvantages of a hybrid sensor are its
cost and reliability. Cost can be several times higher than the electronic-
assembly option discussed next, and reliability testing is difficult to
amortize, so either limited reliability screening is implemented, or
the cost per unit becomes high.

6.8 Sensor Electronic Assemblies
Combining any of the sensors listed earlier with printed-circuit-based
electronic signal processing creates sensor assemblies or black boxes.
The user defines specifications for light input and the desired output
response; the vendor builds and tests the systems to ensure that the
specifications are met. An assembly can also include optical compo-
nents such as lenses and special wavelength filters. The user just bolts
the assembly in place and connects it to the high-level electronics;
there are no concerns about mismatch between the purchased sensor
and front-end amplifiers or diagnostic electronics. The system is rela-
tively immune from noise and is highly reliable because of the mature
manufacturing technologies used.